A targeting vector was designed to replace the first exon of the targeted gene with a cre cDNA sequence and an frt-flanked PGK-neo cassette. The construct was electroporated into 129X1/SvJ-derived RW4 embryonic stem (ES) cells. Chimeric mice were bred to generate mutant mice. Mutant mice were then bred to FLPe mice (C57 genetic background) to remove the selection cassette.